1. Field of the Invention
The present invention relates to a control device of an automatic transmission, and a control method of an automatic transmission.
2. Description of Related Art
Generally, an automatic transmission that transmits the rotation of an engine is mounted on a vehicle. Such an automatic transmission is provided with a torque converter and a transmission mechanism serving as rotation transmission mechanisms. The transmission mechanism is provided with an input clutch for connecting and disconnecting the rotation transmitted via the torque converter from the engine. This input clutch is brought into an engagement state in a case where the automatic transmission is in a forward travel range (hereinafter referred to as “D range”), and is brought into a released state in a case where the automatic transmission is in a neutral range (hereinafter referred as to “N range”).
Therefore, in a case where the automatic transmission is the D range at the time that the vehicle is stopped, the input clutch is in an engagement state. Thus, the load generated by the torque converter becomes large. As a result, the fuel efficiency of the vehicle is worsened. Thus, recently, a control device (hereinafter referred to as “related-art control device”) of an automatic transmission as described in, for example, JP-A-2001-165289, Paragraph No.
FIG. 8, is suggested as a device that can improve the fuel efficiency of a vehicle.
In the related-art control device, in a case where it is determined that the vehicle is in a stopped state, the neutral control for putting the input clutch in an engagement state into a half-engagement state is executed. That is, in the related-art control device, in-neutral control is executed after execution of the release control of causing the oil pressure for the input clutch to be reduced to the oil pressure immediately before the input clutch is brought into a half-engagement state. During the in-neutral control, the related-art control device detects the variation of differential rotation between the engine speed of the input side that becomes the engine side of the torque converter, and the rotating speed of the output side that becomes the input clutch side while the oil pressure for an input clutch is minutely changed. Also, in a case where the variation of differential rotation is within a preset predetermined range, the related-art control device determines that the input clutch is actually in a half-engagement state, and the oil pressure for the input clutch is held at a fixed pressure. Therefore, at the time of the stop of the vehicle, the input clutch is maintained in a half-engagement state. Therefore, the load generated by the torque converter is reduced, and as a result, the fuel efficiency of the vehicle is improved.
Meanwhile, whether or not the input clutch has been brought into a half-engagement state on the basis of the execution of in-neutral control is determined on the basis of whether or not the variation per unit time of differential rotation is within a predetermined threshold range. Therefore, when the in-neutral control is started, the magnitude of the differential rotation may differ whenever the neutral control is executed. Accordingly, in a case where the differential rotation is comparatively large during the in-neutral control, there is the possibility that the calorific value generated in the input clutch in a half-engagement state may be increased as compared with a case where the differential rotation is comparatively small, and the durability of the input clutch might degraded.
Further, even if the oil pressure for the input clutch is held at a fixed pressure during the in-neutral control, and the oil pressure is actually in a stable state, the differential rotation may become large gradually. In such a case, the calorific value generated in the input clutch may be gradually increased as the differential rotation becomes large. As a result, there is a possibility that the durability of the input clutch may degrade.